Detecting rolling element bearing faults with vibration analysis
www.33cm.net www.mobiusinstitute.com
Detecting rolling element bearing faults is the highest priority for most vibration analysts. Detecting the fault at the earliest opportunity should be the priority, however in reality most analysts do not detect the fault in the first or even the second stage of failure. This article is going to help you to detect faults at stage one so that you can truly be in control of your maintenance program.
In this article I will describe the four stages of bearing failure and how to understand and successfully utilize the airborne ultrasound, Shock Pulse, Spike Energy, PeakVue, enveloping/demodulation, time waveform analysis and spectral analysis methods. I will also explain why you should not rely on trending overall level readings.
Reducing bearing faults No article of this nature can be complete without a discussion of the reasons why bearings fail in the first place. Your first priority should be to minimize the causes of bearing failure. If you can do that successfully, then you will not need to rely on the vibration analysis techniques as much. That is not to say that I want to put vibration analyst’s out of work, or that you should even consider downsizing your vibration monitoring program (because there will always be bearing failures and other mechanical faults) – the point is that the path to equipment reliability does not begin with vibration analysis.
The fact is that if you properly purchase, transport, store, install, and lubricate your bearings, and you operate machines that are balanced, aligned and operating well away from natural frequencies, your bearings will last longer.
You may not have control over many of these factors, but if you are involved in vibration analysis then there are two things you can definitely do: look for the presence of conditions that will cause bearings to have a reduced life, and perform root cause analysis when you detect bearing damage.
I opened this article by pointing out that the detection of rolling element bearing faults is the highest priority for most vibration analysts. The sad truth is that for too many analysts it is the only priority. Unbalance, misalignment, soft foot, and resonance often have a much lower priority. Although these faults conditions appear first on most wall charts, they can be the trickiest to diagnose. Phase analysis is a powerful, yet underutilized tool that can greatly help in the detection of these fault conditions – but that was the topic of an earlier article.
The point is that these conditions put additional stress on the bearings, thus reducing their life. If you do not take care of these conditions, it is inevitable that you will soon see the earliest stages of bearing damage.
The pattern of bearing damage Before we get into the specifics of the four stages of bearing failure, I would like to describe how the vibration changes in general terms. In classical teaching, bearing vibration is all about the four forcing frequencies: ball pass outer race (BPFO), ball pas inner race (BPFI), ball spin (BSF), and cage or fundamental train frequency (FTF). We will discuss these in more depth in a moment, but first I want to describe the movement of “broadband energy”.
If a bearing is poorly lubricated, we can detect an increase in the level of “noise” at very high frequencies. It is not a specific, single frequency; instead it will depend on a number of factors to do with the machine’s construction. Suffice to say that you cannot hear it; it is well above your hearing range.
As the state of lubrication worsens, the level of the noise will increase, but the frequency of the noise will slowly reduce – it will move from very high frequencies to high frequencies. That is not to say that you can’t detect the condition at lower frequencies; it is stronger at the higher frequencies.
As the film of lubricant between the bearing surfaces is reduced further, we will have more and more metal-to-metal contact, causing “stress waves” to be generated. Stress waves (also referred to as “shock pulses”) are like ripples in a pond; the moment the metal surfaces make contact, a wave of energy races away from the point of contact at the speed of sound. It all happens very quickly; possibly in less than a thousandth of a second!
Even if the root cause of the bearing fault is not poor lubrication, if the bearings are damaged through poor installation, false brinelling (where the bearing has been vibrating whilst it is stationary), EDM, misalignment, or any one of a number of reasons, there will come a time when there is either metal to metal contact between two surfaces,
